This expansion board for the eMG100 phone system expands it by 8 analogue extensions.  This can be used for analogue phones such as hotel room phones, cordless phones, fax machines etc.  This is the main card, the daughter board SLIU can be attached to this but requires this card installed.  If more than 8 analogue extensions are required and you need to expand past this board, you will need the SLIU8.